Alligator weed is an immersed aquatic plant. It originated in South
America, but has spread to many parts of the world and is considered an
invasive species in Australia, China, New Zealand, Thailand and the United
States. It is a sprawling herb, usually in water, often in row crops and
gardens. Stems are pinkish, can become hollow when larger, up to 3.3 ft
long. Leaves are opposite, narrowly elliptic or spatulate, up to 9 cm
long; occasionally a few indistinct teeth on margin. Flowers are reduced,
bisexual in round white heads on long stalks from upper leaf axils. Each
flower has 4-5 thin, papery bracts, 5 stamens, 1 pistil. Fruit is tiny,
one-seeded, thin-walled.